From Joomla! Documentation

JForm, a feature introduced in Joomla 1.6, lets you easily create HTML forms (<form>). Forms created using JForm consist of form fields, implemented as JFormFields. There is a JFormField for each different field type you can find in a form, such as a text field type and a date field type. JForm supports a large selection of standard field types. For a full list, see Standard form field types.

Joomla 1.6 makes it possible to extend standard field types or define your own. For example, if your component manages phone book entries, you might want to define a form field type that outputs a select list of cities. There are several advantages to defining a custom form field type:

You will be able to mix standard field types with your custom field type in a JForm-based form.

You will eventually have a reusable code package that can be used easily throughout your code.

Extensions that collaborate with your extension will be able to create form fields without meddling with your database tables and other internals.

Form field type class requirements

A form field type is defined in a class that must be a (not necessarily direct) subclass of JFormField. To work correctly, the class must define at least two methods:

public function getLabel()

This function will be called to create the label that belongs to your field and must return a HTML string containing it. Since JFormField defines a ready-to-use getLabel() implementation, custom form field types usually do not define their own getLabel(). If you leave it out, the inherited method of creating labels will be used. It is recommended to leave out the getLabel() method for consistency and speed unless you actually want to modify the label's HTML.

public function getInput()

This function will be called to create the field itself and must return a HTML string containing it. This is also where most of the processing usually happens. In our phone book City field example, this function will have to retrieve a list of available cities and return a HTML <select> with the cities inserted as <option>s.

Inside your code, you will have to process the attributes set by the field's user in the XML form definition. Some of those attributes are accessible via protected member variables of JFormField. For example, the name attribute is available in your code as $this->name. Similarly, label, description, default, multiple and class are also available as properties of $this. Other parameters you might have defined can be accessed through the $this->element array: the attribute size will be in $this->element['size'].

Which class to subclass?

For a form field type to be usable in JForm, it needs do be a subclass of JFormField. However, it does not have to be a direct child of that class: you can also subclass an existing (standard or custom) form field type and thereby inherit useful code.

If your form field type is quite similar to an existing type, you should subclass that type. Especially if your form field type is a list, please subclass JFormFieldList. You only have to override getOptions() method to return the options to be shown; the getInput() method will convert those options to HTML.

To subclass an existing type, for example JFormFieldList, load it by adding the following to after jimport('joomla.form.formfield');:

jimport('joomla.form.helper');
JFormHelper::loadFieldClass('list');

If your form field type is unlike any existing type, subclass JFormField directly.

Location of files

The standard form field types are located in joomla/libraries/joomla/form/fields/. You should not store custom fields there, nor should you have to use this path in your own code, but the standard types are usually good examples.

The custom field types that belong to your component are usually located in administrator/components/<name of your component>/models/fields. You can specify this or another path in your code:

JForm::addFieldPath(JPATH_COMPONENT .'/models/fields');

The XML files that define forms are usually located in administrator/components/<name of your component>/models/forms. Use something like the following snippet to specify a path to your forms:

JForm::addFormPath(JPATH_COMPONENT .'/models/forms');

Naming conventions and skeleton

In this section, <ComponentName> represents the camel-cased name of your component and <TypeName> represents the camel-cased name of your form field type. The field's class should be placed in administrator/components/<name of your component>/models/fields/<name of your field>.php, and look like this:

<?php// Check to ensure this file is included in Joomla!defined('_JEXEC') or die('Restricted access');
jimport('joomla.form.formfield');// The class name must always be the same as the filename (in camel case)class JFormField<FieldName>extends JFormField {//The field class must know its own type through the variable $type.
protected $type='<FieldName>';publicfunction getLabel(){// code that returns HTML that will be shown as the label}publicfunction getInput(){// code that returns HTML that will be shown as the form field}}

Grouping custom field types

Warning: this information is partially incorrect and needs to be improved.

Custom field types can be grouped by using an underscore in the field name. A field class with a name for example like "JFormFieldMy_randomField" must be stored in administrator/components/<name of your component>/models/fields/my/randomField.php. We can prefix our form field names with some group name, then we put an underscore and then a name of a field.

An example custom field type

Suppose you're working on your component named com_phonebook and you want to define a field that contains cities. Create the file administrator/components/com_phonebook/models/fields/city.php and write something similar to the following:

Using the custom field type

Linked with a form

To use the field type City, we need to update the XML file that contains the form fields. Open your XML file located in administrator/components/com_phonebook/models/forms and add the field in the usual way:

Not linked with a form

E.g. when you need the field as a dropdown in a component as admin/site filter.

//Get custom field
JFormHelper::addFieldPath(JPATH_COMPONENT .'/models/fields');$cities= JFormHelper::loadFieldType('City',false);$cityOptions=$cities->getOptions();// works only if you set your field getOptions on public!!

Overriding getLabel()

As mentioned in the section Form field type class requirements, custom form field types usually do not define their own getLabel(). If you do want to create a custom label, you can still make use of the getLabel() that every field type class inherits from JFormField, for example by defining it as follows: